The development of a neuroprotective/restorative therapy for Parkinson's disease (PD) would be a major therapeutic advance. Glial cell line-derived neurotrophic factor (GDNF) has been the most potent trophic molecule tested so far and in numerous studies has clearly demonstrated a capacity to protect and restore neurons affected in PD. However, clinical trials of GDNF in PD patients have given mixed results. Effective targeted delivery to specific brain sites of disease is believed to be the key impediment to consistent success. GDNF requires focal delivery as it does not cross the blood- brain barrier. The precise location of the cannula for delivery is critical and it is challenging to achieve therapeutic levels of GDNF for all or most degenerating neurons due to the relatively large target area in human brain and the poor brain tissue penetration of this molecule. Additionally, current GDNF therapy entails important safety concerns. We recently developed a novel approach - macrophage- mediated GDNF delivery - that seems capable of resolving these problems. This unique approach takes advantage of the well-known macrophage property of homing to degenerating sites in proximity to damaged neurons, capitalizes on our powerful macrophage-specific synthetic promoters (MSP), and implements recent advances in hematopoietic stem cell gene therapy. We hypothesize that highly effective CNS delivery of GDNF can be achieved through its expression in macrophages / microglia by ex vivo transduction of HSC-containing bone marrow cells with lentiviral vectors carrying a cassette expressing GDNF driven by our MSP, followed by syngeneic transplantation of these transduced bone marrow cells, and this will greatly ameliorate the pathological changes and neurological defects of PD. Using a sub-acute MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) mouse model of PD in a neuroprotection paradigm, we demonstrated that genetically engineered bone marrow cell-derived macrophages accumulate in diseased sites and macrophage-mediated GDNF delivery dramatically reduces degeneration of dopaminergic neurons of the substantia nigra, as well as their fibers in the striatum, and induces axon regeneration, without any apparent adverse effects. In order to move this novel concept and unique approach eventually into clinical application, here we propose to explore it in depth in both neurotoxin and genetic mouse models featuring the chronic and progressive changes characteristic of PD. In the chronic toxin model, we will utilize a tetracycline-regulatable (Tet-on) system and address efficacy of our approach by doxycycline-induced switching on of GDNF expression and delivery in a neurorestorative or neuroprotective / restorative paradigm, simulating treatment of PD. In the MitoPark conditional gene knockout model, we will perform transplantation of the transduced bone marrow cells at ages of the mice when they reach the neurodegenerative stages equivalent to preclinical, clinical, and advanced PD. We will also address the safety issue by monitoring for adverse effects, particularly those described in the literature, after short- or long-term GDNF expression, and examining the capability of tapering or shutting off GDNF expression when necessary through withdrawal of doxycycline. This translational study will establish a solid base for future clinical investigation of the potential benefits to patients of this novel neuroprotective therapy for PD.